The invention relates to a device for mixing an input signal having a first frequency RF with an input signal coming from a local oscillator having a second frequency LO, and for producing an output signal having an intermediate frequency IF, with rejection of the harmonics of the second frequency LO up to a very high order, for example, higher than the tenth order, this circuit comprising a ring mixer of double balanced field effect transistors.
The invention finds its application in the integrated circuit industry for the field of telecommunications and, more particularly, in the industry of monolithic microwave integrated circuits (MMICs).
A device including a double balanced and monolithically integrated mixer having a high third order intercept point is already known from the state of the art via U.S. Pat. No. 5,060,298 (referenced Waugh, Oct. 22, 1991).
This document describes a radio frequency mixer which converts a frequency RF of an input signal into an intermediate frequency IF of an output signal by adding together the frequency RF of said input signal and the frequency LO of a signal produced by a local oscillator.
For establishing the mixer itself, four field effect transistors without drain-to-source bias are arranged in rings. The transistors are mounted so that a source of a transistor is connected to the source of the adjacent transistor on one side, and the drain of said transistor is connected to the drain of the adjacent transistor on the other side. The gates of the non-adjacent transistors are coupled to each other.
The mixer also comprises an impedance transformer circuit (BALUN), a frequency combiner circuit (COMBINER) and a phase splitter circuit (PHASE SPLITTER). The impedance transformer circuit comprises at least six distributed stages of field effect transistors and is supplied with the signal coming from the local oscillator to produce two signals having the frequency LO in phase opposition, applied to the coupled gates of the transistors of the ring mixer. It is highly significant to ascertain that this impedance transformer circuit does not have identical impedances on the two anti-phase output terminals directed to ground, respectively (FIG. 3 of said document). The combiner circuit comprises two distributed stages and is connected to the coupled sources of the ring mixer to combine the output signals having the anti-phase intermediate frequency IF. It is highly significant to observe that for combining the signal IF with its complementary signal, this circuit cannot offer the same impedances on its two input terminals, otherwise the combined signal will be zero. The phase splitter circuit comprises a double gate transistor stage and produces on the basis of an input signal having the frequency RF, two anti-phase signals having the frequency RF to be applied to the coupled drains of the ring mixer. Similarly to the other circuits connected to the mixer, this phase-splitter circuit cannot present identical impedances on its output directed relative to ground, otherwise the output signals would be zero.
This known device has for its object to provide an intercept point IP3 of a higher level than 30 dBm. To achieve the selected aim, this mixer circuit is double balanced, i.e. receives anti-phase signals on the two terminals formed by the coupled drains of the transistors and produces anti-phase signals on the two terminals formed by the coupled sources of the transistors; similarly, the gates of the pairs of crosswise coupled transistors receive the signals in phase opposition. The signals having the frequencies RF and LO are situated in a narrow frequency domain between 8.5 and 10.5 GHz for producing a signal having the remote intermediate frequency IF from 0.5 to 2 MHz.
The device known from the cited document is only interested in the third harmonic. Generally, a person of ordinary skill in the art knows that the value of the intercept point IP3 for the greater part depends on the ring mixer and that ever since field effect transistors have been used for establishing the ring mixer, the value of this intercept point is rather high. This value also depends on transforming ratios of the impedance transformers.
A technical problem is posed when the input signal having the frequency RF and the output signal having the intermediate frequency IF are situated in a very narrow frequency domain, instead of the signal having the frequency RF and the signal having the local oscillator frequency as in the cited document. For producing a signal having the intermediate frequency IF which substantially approximates the frequency RF of the input signal, the local oscillator signal is thus to have a frequency LO which is notably lower than that of the signal having the frequency RF so as to obtain the desired frequency mixture.
A person skilled in the art knows that harmonics are always obtained in whatever mixing system. When the signal of the local oscillator has a frequency that is lower and very different from that of the input and output signals i.e. RF and IF which are, on the one hand, higher and, on the other hand, not very different from each other, then the result is that higher-order harmonics, for example, harmonics of the tenth or eleventh order of the local oscillator signal having the frequency LO are situated in the narrow frequency domain of the input and output signals having the respective frequencies RF and IF.
The elimination of the harmonics having the frequency LO of the local oscillator, which run at least up to the tenth order, is thus a different problem from the problem discussed by said document. This problem can only be solved by specific means.
Another problem is posed nowadays due to the demand from the market and the state of the competition, which resides in the fact that only the circuits consuming the least energy are purchased. From this point of view, the device according to the cited document is unfavorable, because it consumes much energy owing to the fact that the circuits connected to the mixers are active circuits which include many transistor stages.